1. Field of the Invention
The present invention relates to cutting tools which have a cutting, grinding, drilling, etc., surface formed of polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN). More specifically, the present invention relates to such tools in which an intermediate layer is disposed between the substrate and polycrystalline outer layer to extend the useful life of the cutting tool.
2. State of the Art
In the application of cutting, turning, drilling and machining tools, both tool wear and production throughput rates are of critical importance. If a tool is highly wear resistant, but can only be used at low throughput rates, its usefulness is significantly limited, as the profitability of many industries depends on being able to produce a large number of pieces with as little machinery as possible. Likewise, if a tool allows for a high throughput rate but does not have significant wear resistance, the time savings of the high throughput is often negated by the down-time and expense associated with replacing the tool.
Because of their significant resistance to wear, industries which utilize cutting, grinding, machining, turning and drilling tools typically use tools having PCD or PCBN segments. The hardness of the diamond or cubic boron nitride enables the tools to cut through considerable amounts of material with relatively little wear.
In forming a conventional diamond or cubic boron nitride tool, a layer of the polycrystalline material is bonded to a support material such as a tungsten carbide substrate. While diamond and cubic boron nitride are super hard and good thermoconductors, tungsten carbide is a strong shock and heat absorber, and is also a relatively strong and ductile body. Typically, the tungsten carbide contains about thirteen percent cobalt.
The polycrystalline material, the cobalt and tungsten carbide support are then subjected to a high pressure, high temperature manufacturing method such as that disclosed in U.S. Pat. No. 3,745,623 to Wentorf. In that patent, Wentorf describes a diamond sintered PCD integrally bonded to a strong tungsten carbide substrate for diamond machining applications. U.S. Pat. No. 3,743,489, also to Wentorf, discusses making a similar tool with a cubic boron nitride powder resulting in a PCBN compact.
While PCD and PCBN tools are extremely wear resistant, they are prone to becoming extremely hot under high throughput situations. For example, in Industrial Diamond Review (Vol. 56, No. 569, Page 40) A.M. Abrao documents that tool temperature increases proportionally with the speed of cutting. Thus, a PCBN cutting tool working against hot work die steel can exceed 800.degree. C. at 200 m/min. PCD also behaves likewise. The higher the throughput of the element to be cut, ground, drilled, etc., the higher the temperatures the cutting tool will reach.
While PCD and PCBN are extremely durable materials, tools using these materials are subject to heat degradation. Unlike diamond or cubic boron nitride which have a high degree of thermal conductivity, cobalt has a high degree of thermal expansion. Thus, while the diamond or cubic boron nitride expands very little as temperature increases, the cobalt's expansion can be significant. Additionally, the cobalt can serve as a catalyst for reactions, such as the pseudo-oxidation reaction by which diamond particles can be converted to graphite. Furthermore, excess migration of the cobalt from the tungsten carbide can also result in weakness at the upper layer of the substrate, possibly causing separation between the substrate and the diamond particles.
The thermal expansion and catalytic effects of the cobalt would be less of a concern if the cobalt remained exclusively at the interface between the polycrystalline material and the tungsten carbide substrate. However, it is well known that cobalt tends to migrate from the tungsten carbide substrate and infiltrate the polycrystalline compact during the high pressure, high temperature manufacturing process. The migration of the cobalt is not quantitatively controllable and currently available tools, such as General Electric's BZN-6000 PCBN cutter, contain significant amounts of cobalt in the PCBN compact even though cobalt is known to be an improper PCBN binder except for ensuring suitable bonding between the PCBN and the tungsten carbide substrate. Thus, it is difficult to optimize the polycrystalline material for desired tool performance. As the polycrystalline material heats up due to friction with the piece worked, the cobalt expands and destabilizes the polycrystalline compact, and/or serves as the catalyst for graphite formation. Either of these, in turn, causes the PCD or PCBN structure to be prone to failure. The failure may be the result of PCD layer delamination, deterioration of the microstructure, or tool chipping. Regardless of the cause, the tool must be replaced and productivity is lost during the resulting down-time.
Thus, industry is placed in the dilemma of utilizing the high wear resistant PCD or PCBN tools and limiting the speed at which items are worked, or running the tools at a high throughput rate and allowing high failure rate. As the economics of high throughput become increasingly important, those in industry must continue to find ways to improve both the wear resistance and heat tolerance of such tools.
Thus, there is a need for improved PCD and PCBN tools which are better able to provide significant wear resistance, while simultaneously resisting failure under high throughput, high temperature conditions. Such tools should be relatively easy and inexpensive to make. Such tools should also be relatively inexpensive and easy to use.